Spray gun

ABSTRACT

WORK IS ELECTROSTATICALLY COATED WITH HIGHLY CONDUCTIVE MATERIAL BY HYDROSTACIS ATOMIZING SPRAY GUNS. THE RATE OF MATERIAL APPLICATION TO WORK FOR WATER BASE MATERIAL SUCH AS PORCELAIN ENAMEL IS HIGH BEING IN EXCESS OF THREE MILS OF FILM THICKNESS IN LESS THAN SIXTY SECONDS TO PREVENT &#34;DRY SPRAY.&#34; THE PROBLEM OF &#34;DRY SPRAY&#34; IS FURTHER CONTROLLED BY PLACING THE SPRAY GUNS VERY CLOSE TO THE WORK (LESS THAN TWELVE INCHES).

Nov. 23, 1971 A. c. WALBERG SPRAY' GUN Original Filed Sept. 16. 1966lmvempa a? C //& w g AifORNEYS Nov. 23, 19M A. c. WALB-ERG SPRAY GUN 2Sheets-Shcet 2 Original Filed Sept. 16. 1966 INVENTOR z gez ATTORNEYSUnited States Patent 3,621,815 SPRAY GUN Arvid C. Walberg, Lombard,lll., assiguor to Graco, Inc., Minneapolis, Minn.

Original application Sept. 16, 1966, Ser. No. 580,468, now Patent No.3,463,121, dated Aug. 26, 1969. Divided and this application Aug. 22,1969, Ser. No. 852,253

Int. Cl. B0511 /02 [1.5. Cl. 118-631 14 Claims ABSTRACT OF THEDISCLOSURE Work is electrostatically coated with highly conductivematerials by hydrostatic atomizing spray guns. The rate of materialapplication to work for water base material such as porcelain enamel ishigh being in excess of three mils of film thickness in less than sixtyseconds to prevent dry spray. The problem of dry spray is furthercontrolled by placing the spray guns very close to the work (less thantwelve inches).

This is a divisional application of US. patent application Ser. No.580,468, filed Sept. 16, 1966, now Pat. No. 3,463,121, issued Aug. 26,1969.

The present invention relates to spraying systems for applying coatingmaterial and more particularly to systems which are capable ofelectrostatically depositing coating materials having water as acarrier.

Attempts to spray porcelain enamel electrostatically using water as acarrier have met with failure during the past fifteen years. The qualityof finish has been unsatisfactory. The various electrostatic systemsutilized causes defects such as (1) dry spray, (2) insufficient filmthickness, (3) poor atomization, (4) non-uniform film thickness, (5)incomplete coverage and (6) poor electrostatic wrap-around.

Dry spray was caused by building the film thickness too slowly or byhaving the atomized spray travel excessively long distances before it isdeposited on the surface of the ware. The basic design of the systemsused prevented the correction of the above problem. Since the liquidused in the poreclain enamel solution is water, the wetness or drynesscannot be adjusted in the same manner that solvent adjustments are madein electrostatic painting systems. When dry spray is encountered inpainting, a slower wetter solvent can be added to the paint mixture. Apaint that is too wet" can be corrected by adding a faster solvent. Thisadjustable feature is missing when water solutions are sprayedelectrostatically and this factor prevented prior art electrostaticsystems from properly applying water base materials.

Insufiicient film thickness was applied in previous attempts to masterthe spraying of porcelain enamel electrostatically. A heavy paint filmfalls in the 1.0 to 1.5 mils range of film thickness. Equipment that wasdesigned to apply films of this thickness was not adequate to meet the3.0 to 6.0 mils of thickness specified for porcelain enamel. Pooratomization of the porcelain enamel occurred when attempting to atomizethe larger volumes of material needed to apply the heavy film thicknessindicated above.

Non-uniform film thickness was caused by stacking several electrostaticspray guns in a vertical plane to spray ware that was wider than the 12inch maximum that could be handled by one gun. It was difficult if notimpossible to mate the adjacent spray patterns together to avoid animproper boundary effect. If the adjacent spray patterns overlapped,there was a heavy build-up of enamel in the overlapped area. If thespray patterns fell short of meeting, a light streak could occur. Theresulting quality was not acceptable.

Some attempts were made to overcome the above problem by oscillating thespray guns in a vertical plane. This moved the heavy and light streaksaround somewhat, but it did not correct the basic problem.

Incomplete coverage encountered with previous electrostatic systems wasdue to the excessive amount of waste caused by the poor operatingefliciency of these systems. Some systems had better chargingcharacteristics and wasted less porcelain enamel but could not penetrateinto recessed areas, corners, or other hard-to-reach areas. The netresult was incomplete coverage and excessive amounts of manual touch-upto complete the job. The actual savings received, if any, was slight.

Poor electrostatic wrap-around caused by previous attempts in thespraying of porcelain enamel electrostatically resulted in poor coverageof the ware. The air spray guns used required excessively high atomizingair pressures to atomize the high volume of porcelain enamel that had tobe sprayed. This caused excessive spray velocity and reducedelectrostatic wrap-around. Disc-type atomizers, on the other hand,cannot atomize electrically conductive materials by electrostatic forcesalone and had to be operated at exceptionally high rotational speed toatomize the porcelain enamel centrifugally. This again resulted in highvelocity that caused poor electrostatic wrap-around. For the abovereasons, electrostatic systems that have proved successful in applyingpaints and similar coatings have proved unsuccessful when used to applyporcelain enamel.

One system used consisted of a standard air-atomizing spray gun sprayingthrough an ionizing electrode charged to 100,000 volts at the groundedware passing on a conveyor at right angles to the direction of thespray. The ionizing electrode consisted of two vertical tubes mounted12" from the surface being sprayed and on 20" centers. A row of needlepoint ionizers were attached on 2" centers to the tubes on the sidefacing the ware. The needles pointed toward the ware. A spray gun or astation of several spray guns mounted on a vertical plane were arrangedto spray between the two vertical electrodes. The guns were at groundpotential and were positioned 12" behind the ionizing electrodes orabout 24" from the grounded surface being sprayed. This 24" distance oftravel of the atomized spray particles caused excessive drying of thespray particles resulting in a dry coat being applied to the product.There was insufficient flow-out. The quality of finish was notacceptable. In addition, all of the aforementioned faults wereencountered in this system.

A variation of the above system called for mounting the guns to spraysubstantially parallel to the ware moving along on the conveyor. Thespray passed between an ionizing electrode and grounded ware and wasdeposited on the ware by the electrostatic field. The distance traveledby the spray was again excessive and the resulting finish wasunsatisfactory due to dry spray. All of the other faults indicated abovewere also encountered.

Both of the above ssytems were also unsuccessful because of lowoperating efficiency. Too much enamel was wasted.

A newer electrostatic system consisted of a horizontal spinning discreciprocating in a vertical plane with a conveyor carrying the ware in aloop around the disc. The disc was charged to 100,000 volts D.C. and theporcelain enamel was distributed over the disc by centrifugal force tobe atomized at the edge of the disc. The disc normally atomizes paintelectrostatically but it will not atomize electrically conductivematerial electrostatically. Since poreclain enamel is electricallyconductive, it was necessary to drive the disc at high speed to atomizethe porcelain enamel by centrifugal force. This system was moreefiicient since it atomized the material at the zone of ionization butthe material was applied slowly as the ware passed around a 20 footconveyor loop. This caused dry spray and required complete respraying ofthe surface with conventional air-atomizing spray guns to obtain anacceptable quality.

The disc type atomizer also has severe limitations on the volume ofporcelain enamel that could be atomized per unit of time. This greatlylimited material delivery and caused insufiicient film thickness andrequired excessive manual touch-up to complete the job.

A disc type atomizer had an additional disadvantage since it sprayed atall empty spaces between the products on the conveyor and causedexcessive waste and uncontrolled buildup of coating on edges of aproduct that happened to be adjacent an open space.

The present invention overcomes a major share of the problemsencountered in previous systems. Dry spray is not a problem. It wasovercome by placing specially designed electrostatic spray guns at amuch closer distance to the surface being sprayed. A spacing of 8" towas found satisfactory. This spacing would be unsafe if a solvent-basedpaint was used in the system. The entire film of several mils is appliedin a relatively short distance of conveyor travel. The actual coatingoperation is complete in six feet of conveyor travel or less. Thevelocity range of conventional conveyors is from ten feet per minute totwenty four feet per minute. Thus, the present invention applies therequired film thickness in less than sixty seconds. This assures thedesired wetness and the desired quality of finish.

Therefore, it is a principal object to provide a new and improvedelectrostatic spraying system for spraying abrasive material.

Another object of the present invention is to provide an electrostaticspraying system which is capable of effectively spraying porcelainenamel or other highly abrasive material that requires control of thetemperature of the atomizing air so as to counteract the effect ofhumidity in the atmosphere and thereby control wetness or dryness of theapplied film.

Further objects and advantages will become apparent from the followingdetailed description taken in connection with the accompanying drawings,in which:

FIG. 1 is a perspective view of an automatic electrostatic sprayingsystem embodying the present invention;

FIG. 2 is a partial sectional drawing of the electrostatic spray gunillustrated in FIG. 1; and

FIG. 3 is a partial broken-away elevational drawing of the electrostaticspray gun illustrated in FIG. 2 taken alone the line 4-4.

While this invention is susceptible of embodiment in many differentforms, there will be described herein in detail an embodiment of theinvention with the understanding that the present disclosures are to beconsidered as exemplifications of the principles of the invention andare not intended to limit the invention to the embodiment illustrated.The scope of the invention will be pointed out in the appended claims.

Referring now specifically to FIG. 1, a conveyor 40 of any conventionaldesign well known to those skilled in the art propels work to be coated,such as panels 41 and 42, when they are connected to the conveyor 40 bymeans of conductive hangers 43 and 44, respectively. The conveyor 40 iselectrically connected to an electrical ground and therefore the workcarried by the conveyor, such as panels 41 and 42, are also connected toground through the respective electrically conducting hangers, such as43 and 44, and the conveyor '40. A hydraulic reciprocator generallyindicated at 50 has a base 51 upon which is mounted a hydraulic unit 52and vertical guide rails 53 and 54. A pair of pulleys 55 and 56 arerotatably mounted to the guide rails 52 and 54 near the top and bottomof the rails. The pulleys 55 and 56 have grooves shaped to receive adrive chain 57 which has its ends connected to a spray gun carriage 58.The carriage 58 has ball bearings mounted internally (not shown) whichcontact the respective rails 53 and 54 in order that the carriage 58 maybe smoothly guided in vertical movement by the guide rails. An analogcontrol generally indicated at 60 is operatively connected to the chain57 and has a pair of cams 61 and 62 for engaging a rabbit ear hydraulicfour-way valve 63. The analog control 60 and the fourway valve 63reverse the direction of travel of the spray gun carriage at the top andbottom of its stroke. By moving the cams closer together the unit willtake a short stroke. The shortest stroke will be in the neighborhood of12 inches. By moving the cams on the analog control further apart, thelength of the stroke is increased up to the maximum possible for thesize of the reciprocator. The maximum stroke is approximately three feetless than the height of the reciprocator 50. The four-way valve 63 isconnected to the hydraulic unit 52 by hydraulic lines 64 and 65. Anotherpair of hydraulic lines 66 and 67 are connected between the hydraulicunit and a hydraulic drive motor 68 which is connected to the pulley 56to drive it, and thereby drive the chain 57 and the spray gun carriage58. Since the hydraulic unit 52 may be any conventional source ofhydraulic pressure well known to those skilled in the art, and since thevalve 63 and drive motor 68 are connected and operated as is well knownto those skilled in the art, this structure will not be described indetail.

A hollow cross arm 70 in the form of a tube of electrically insulatingmaterial is rigidly secured in the spray gun carriage 58. The movementof the cross arm 70 is therefore controlled by the analog control 60.The speed of reciprocation is adjusted by a manual control 69 which ismounted on and connected to the hydraulic unit 52. A set of four sprayguns 71-74 are clamped to the cross arm 70.

A high voltage power supply capable of supplying voltages generally inthe range of 60,000 to 150,000 volts has its positive output terminalconnected to electrical ground and its negative output terminalconnected to the spray guns 71-74 by high voltage electrical lines 81,82, 83, and 84. The high voltage line 83 passes through the hollow crossarm 70 in order to reach gun 74 and the gun 73 via line 84.

A porcelain enamel pressure tank is mounted on a three-legged insulatorstand 91. The tank 90 has a cover 92 which makes an air-tight seal forthe tank 90 when screw clamps 93, 94, 98 and 99 are utilized to secureit. A source of air pressure (not shown) is connected to an air line 95of electrical insulating material (such as nylon) which has a manuallyor remotely operated air regulator 96 placed therein. The air line 95 isconnected to the tank cover 92 to supply air at controlled pressure tothe interior of the tank. A fluid hose 97 of electrical insulatingmaterial (such as nylon) extends through the cover 92 downward into thetank 90 and is connected to each of the guns 71-74 to supply coatingmaterial thereto. It will be noted that the hose 97, after it suppliescoating material to the guns 71 and 72 through short connectingbranches, passes through the hollow cross arm 70 to reach guns 73 and74.

An air heater 100, which contains electrical heating elements (notshown), has these elements electrically connected to a source ofelectrical potential (not shown) by electrical leads 101 and 102. Theair heater has its air passages connected to a source of high pressureair (not shown) through an air line 103, a manual air pressure regulator104, and an air line 105 and connected to the gun 74 by an air line 106.A similar atomizing air heater (not shown) is provided for each of theother spray guns. A solenoid air valve is also connected to a source ofhigh pressure air (not shown) by an air line 111, and it is connected tothe trigger mechanism of gun 74, as will presently be described ingreater detail, through the air hose 112. Similar controls (not shown)are provided for spray guns 71, 72 and 73 as well.

Since each of the spray guns 71-74 is identical in construction, onlygun 71 will be described in detail. Referring now to FIGS. 2 and 3, thespray gun 71 comprises a metallic rear housing and a barrel portion ofnylon insulating material, generally indicated at 121, which comprises afirst barrel section 122 threaded to a second barrel section 123 and anair cap 124 secured to the second barrel section 123 by a threadedannular retainer 125. The barrel section 122 is secured to section 120by an annular fitting 126 to which an annular retainer 127 is secured bythreads, and an annular cylinder 128 which is threaded onto section 122and has an annular edge which contacts an annular edge of the retainer127. A stainless steel valve 130 closes a passage 141 through the barrelportion 121 and the rear housing 120 when it engages an annular taperingvalve seat 131 in a fluid tip 142. The valve 130 is connected tostainless steel square shaft 129 which is connected to a stainless steeloperating shaft 132 which is in turn connected to a stainless steelshaft 133. Shaft 133 is connected to an adjustable needle plunger 134which has a spring 135 urging it towards the forward portion of thespray gun. A second spring 136 bears against a spring retainer 137 whichis rigidly mounted on the shaft 132 to urge the shaft 132 and the valve130 forwardly, closing the valve against the valve seat. Water carriedporcelain enamel coating material from hose 97 enters an aperture 140that connects with the passage 141 containing the shafts 129, 132 and133 in order that coating material may flow around the shafts to thefluid tip 142. Thus, the coating material is conducted directly aroundthe shafts which operate the valve in the front of the gun to the valve130 within the fluid tip 142. Preferably, all parts with which abrasivecoating material contacts such as water based porcelain enamel areconstructed of abrasive resistant material such as tungsten carbide andstainless steel. The fluid tip 142 is secured between the air cap 124and a fluid tip housing 143. Sealing rings 144, 145 and 146 preventfluid from the passage 141 from leaking past the fluid tip housing 143.The fluid tip 142 and the fluid tip housing 143 are constructed oftungsten carbide and stainless steel, respectively, so the abrasivecoating material such as porcelain enamel will have a minimum erosioneffect upon these parts. The high voltage electric lead 81 is connectedthrough an electrical lead bushing 146 and an electrical truss screw147. The air hose 106 is connected to the threaded fitting 148 to supplyair to the passages generally indicated at 149 and 150. The passage 150supplies air to two air horns and 161 which form part of the air cap124, and the passage 149 supplies air to an annular air passage 152 inthe fluid tip 142.

The forward edge of the fluid tip 142 and a needle 170 mounted on thevalve 130 and extending through the orifice of the fluid tip 142 form anelectrode.

The length of the overall barrel 121 is normally of the order of 18inches which prevents any substantial electrostatic field from existingbetween the charged rear housing 120 and grounded product 41. Thestrongest electrostatic field exists between fluid tip 142 and needle170 toward grounded product 41.

A pilot bushing aids in completing the passage 141 across the junctionbarrel section 122 and 123, and a cylinder adapter 162 aids incompleting this passage across the junction of the rear housing 120 andthe barrel section 122.

The guns 71-74 are similar in construction and design to the hand gunshown in my US. Pat. 3,251.551, issued May 17, 1966, with the primarydifferences being that harder metals are use for such parts as the fluidtip 142 to resist abrasion, and that electrical connection is madedirectly to the fluid tip through an aperture in the side of the barrelrather than having an electrical lead run from one end of the insulatedportion of the gun to the other end. While the gun illustrated in my US.Pat. 3,251,551, issued May 17, 1966, can be utilized for coatingmaterial having a resistivity of ten megohms or greater per inch cube,the guns illustrated in FIGS. 1-3 can spray electrostatically conductivematerials such as water based materials and such as Teflon. When acoating material such as Teflon is sprayed, it is not necessary toutilize the air heater 100 in the system.

However, in spraying porcelain enamel, the air heater 100 is animportant element of the system. Since porcelain enamel coatings requirethe use of water as a vehicle in the slip being sprayed, the relativehumidity in the ambient air in the spray booth or spray area createsspecial problems in control of the operation. These problems areespecially noticeable during the summer months when the variations inrelative humidity are the greatest. The heater 100 increases theatomizing air temperature as the humidity of the ambient air increases.This evaporates moisture in the sprayed film of coating on the work atan increased rate to permit control of the wetness or dryness of theapplied film. The operating temperature of the heater may be as high asthe order of 210 F. The heater 100 can be either controlled by amanually operated control or by a humidistat. If a humidistat isutilized, a completely automatic system is achieved since the relativehumidity in the spray area or booth can vary substantially andrepeatedly without a resulting deterioration of the quality of theapplied porcelain enamel.

Porcelain enamel is very abrasive and creates problems in regulatorsused to control its pressure. Standard pressure regulators presentlyused in the spray painting art will not stand up under porcelain enamelservice. Therefore, the system illustrated in FIGS. 1-3 does not have aregulator between the tank and the spray gun, but instead utilizes apressurized tank to eliminate the requirement of a regulator. Byutilizing a stationary pressure source, there is a variation in thestatic head at the various gun elevations as the reciprocator raises andlowers the guns as the work passes through their spray patterns asillustrated in FIG. 1. This problem is overcome by the utilization of anorifice that creates sufiicient back pressure at the desired deliveryrate to cause a relatively high pressure to be maintained at thepressure tank 90 containing the porcelain enamel. Thus, the static headbecomes a small percentage of the total fluid pressure at the spray gun.The static pressure of porcelain enamel varies approximately 0.65 to0.75 p.s.i. for every foot of variation in elevation. When reciprocatingthe gun vertically, the gun will deliver more material at the bottom ofthe stroke and less at the top of the stroke. The film thickness on theproduct will vary in the same proportion. When a spray gun is positioned36 inches above the floor, for example, a pressure at the gun of threep.s.i. can be established. If the pressure is controlled by a stationarypressure tank, this pressure will remain fixed. When the gun is raised12 inches, the prressure will decrease approximately 0.75 p.s.i. leavingonly 2.25 p.s.i. At-60 inches above the floor, the pressure will bereduced to 1.5 p.s.i. Thus, with only a two-foot stroke, there would bea pressure variation of 2 to 1 and the applied film thickness would varyin a ratio of approximately 2 to 1. By using a fluid orificesufiiciently small to cause a high tank pressure, the variation in filmthickness from top to bottom of the panel will be greatly decreased. At10 p.s.i. fluid pressure variation of 2 feet in elevation will stillcause a variation in static head of approximately 1.5 p.s.i. This isonly 15 percent of the total and is therefore more acceptable. At 15p.s.i. tank pressure, the variation drops to 10 percent and isconsidered highly satisfactory. Therefore, a tank pressure of 15 p.s.i.or higher is utilized to maintain a substantially consistent filmthickness.

Spray guns 71-74 have the ability to atomize much larger volumes ofmaterial than anything encountered in the past. This permits theapplying of heavy film thicknesses at high operating speed (3.0 mil orgreater in less than sixty seconds) with a minimum amount of equipment.The fast build-up of film thickness also helps to provide the necessarywet coat and eliminates the problem of dry spray. With the guns mountedhorizontally and reciprocated vertically for the full height rather thanbeing stacked vertically as in previous systems, each gun can apply aminimum of two light coats of porcelain enamel. Therefore, the four gunscan apply eight light coats in rapid succession. Thus, the systemillustrated in FIGS. 1-3 can provide higher quality than was possibleprior to the present invention.

While the spraying of porcelain enamel and Teflon in a \vaterbasecarrier has been specifically referred to, those skilled in the art willrecognize that other coatings composed of particles such as variousglass materials can be handled in the same manner as poreclain enamel,and that other particle coatings such as a plastic may be handledwithout humidity control in the same manner as Teflon. While abrasivematerial such as porcelain enamel can only practically be handled by airatomizing spray guns as previously described herein, less abrasivematerials such as Teflon and similar material can be sprayed as hereindescribed both by air and hydrostatic spray guns. The scope of theappended claims is intended to include all such variations andmodifications.

If it is desirable in a particular installation to have an electrodewhich will be highly erosion resistant beyond the structure illustratedin FIG. 2, the fluid tip 142 may be made of tungsten carbide or othersimilar erosion resistant material, and the needle 170 may be laterallyoffset from the nozzle of the fluid tip sufiiciently so that materialbeing ejected from the nozzle of the fluid tip does not impinge upon theneedle. Such lateral offsetting of the needle 170 is accomplished bysecuring the needle to the fluid tip 142 rather than to the valve 130.Such modifications are intended to be within the scope of the appendedclaims.

I claim:

1. An electrostatic coating material spraying system for porcelainenamel frit comprising:

an automatic hydrostatic atomizing spray gun having a nozzle,

mounting means supporting said gun and electrically insulating it fromground potentials,

a container receiving a supply of poreclain enamel frit,

another mounting means supporting said container and insulating it fromsaid ground potentials,

a conduit connecting said container to said spray gun,

means for moving work to be coated relative to said spray gun at apredetermined rate,

means operatively connected to said container, conduit and spray gun toforce coating material from said container through said conduit and saidspray gun nozzle at a rate sufficient to place a film of said materialhaving a thickness of at least 3.0 mils on work to be coated as it ismoved relative to said spray gun at said predetermined rate, and

high voltage power supply means connected between said gun and work tobe coated to provide an electrostatic field between work to be coatedand said gun.

2. An electrostatic coating material spraying system in accordance withclaim 1, wherein all metal parts in said spray gun which contact saidfluid material are constructed of abrasive resistant material.

3. An electrostatic coating material' spraying system in accordance withclaim 1, wherein said mounting means reciprocates said spray gunvertically for at least the full height of work to be coated.

4. An electrostatic coating material spraying system for porcelainenamel frit comprising:

a multiplicity of automatic hydrostatic atomizing spray guns havingnozzles,

mounting means supporting said guns in a horizontal line and electricalyinsulating them from ground potentials, said mounting meansreciprocating said spray guns vertically for at least the full height ofwork to be coated,

a container electrically insulated from ground potentials receiving asupply of porcelain enamel frit,

a conduit electrically insulated from ground potentials connecting saidcontainer to said spray guns,

means for moving work to be coated relative to said spray guns at apredetermined rate,

means operatively connected to said container, conduit and spray guns toforce coating from said container through said conduit and spray guns ata rate sutficient to place a film of said material having a thickness ofat least 3.0 mils on work to be coated as it is moved relative to saidspray guns at said predetermined rate, and

high voltage power supply means connected between said gun and work tobe coated to provide an electrostatic field between work to be coatedand said gun.

5. An electrostatic coating material spraying system for conductivefluid material comprising:

a hydrostatic atomizing spray gun,

fluid coating supply means connected to said gun,

a source of compressed gas,

temperature control means,

means for operatively connecting said temperature control means betweensaid source of compressed gas and said gun to control the temperature ofa flow of gas from said source at its ejection from said spray gun,wherein said temperature control means is a heating means for raisingthe temperature of said flow of gas before its ejection from said spraygun, and

a humidistat connected to said heating means to automatically vary thetemperature of said flow of gas in response to variations in thehumidity of air surrounding work being coated by said spraying system.

6. In combination with the electrostatic coating material sprayingsystem as specified in claim 5:

a mounting means connected to said spray gun for reciprocating saidspray gun vertically for at least the full height of work to be coated,and

a pressure means operatively connected to said spray gun to forcecoating material through said gun under a pressure of at least 15 p.s.i.

7. An electrostatic coating material spraying system in accordance withclaim 6, wherein said fluid coating supply means is a containerelectrically insulated from ground potentials.

8. An electrostatic coating material spraying system for porcelainenamel frit comprising:

an automatic hydrostatic atomizing spray gun having a nozzle,

mounting means supporting said gun at a distance less than twelve inchesfrom work to be coated whenever the gun is operated to coat the work andelectrically insulating said gun from ground potentials,

a container receiving a supply of porcelain enamel frit,

another mounting means supporting said container and insulating it fromground potentials,

a conduit connecting said container to said spray means for moving thework to be coated relative to said spray gun at a predetermined rate,

means operatively connected to said container, conduit and spray gun toforce coating material from said container through said conduit and saidspray gun nozzle at a rate suflicient to place a film of said materialhaving a thickness of at least 3.0 mils on the work to be coated as thework is moved relative to said spray gun at a predetermined rate, and

high voltage power supply means connected between said gun and the workto be coated to provide an electrostatic field between the work and saidgun.

9. An electrostatic coating material spraying system in accordance withclaim 1, wherein said film has a thickness of at least 3.0 mil appliedto said work to be coated in less than sixty seconds.

10. An electrostatic coating material spraying system in accordance withclaim 8, wherein said film has a thickness of at least 3.0 mil appliedto said Work to be coated in less than sixty seconds.

11. An electrostatic coating material spraying system in accordance withclaim 1, wherein said material comprises porcelain enamel frit andwater.

12. An electrostatic coating material spraying system in accordance withclaim 4, wherein said pressure means maintains a fluid material pressuregreater than 15 p.s.i. in said gun.

13. An electrostatic coating material spraying system in accordance withclaim 4, wherein the variation in the static pressure of the fluidmaterial in said gun does not exceed fifteen per cent of the maximumpressure utilized References Cited UNITED STATES PATENTS 2,736,6712/1956 Ransburg et a1. 1l8631 X 2,764,125 9/1956 Juvinall 118-626 X2,780,565 2/1957 Iuvinall l18626 X 2,893,894 7/1959 Ransburg 239-15 UX3,169,882 2/1965 Juvinall et a1 2393 X 3,169,883 2/1965 Juvinall 239-3 X3,463,121 8/1969 Walberg 118631 MERVIN STEIN, Primary Examiner US. Cl.X.R. 23915

